This invention relates generally to aperture stops for cameras or imaging sensors and, more particularly, to variable aperture stops for pixelated imaging array sensors.
To date, many camera or visions systems have been proposed for vehicular use, such as rearview vision systems or the like. The systems typically comprise a lens and camera for providing a focused image of a scene around a vehicle to the driver of the vehicle. The lenses typically comprise multiple optic elements, which are generally desirable to enhance the image quality to a desired level of resolution. Because these multiple element lenses may significantly add to the overall cost, size and weight of the system, it is highly desirable to minimize the number of optical elements that are necessary to focus an acceptable image onto the camera, in order to keep the costs low for such vehicular systems. However, this may lead to a reduction in image quality or resolution.
Furthermore, in order to obtain the desired image quality, a vision system may need to function at an f-ratio (f-ratio=focal length/entrance pupil diameter) of 1.5 or more, in order to properly control lens aberrations. High f-ratios increase the depth of field or focus range of a system but significantly limit the amount of light received by the camera. Low f-ratios allow more light into the system but may cause significant degradation in image sharpness depending on the optical system in use. Operating optical systems with high f-ratios may be adequate in daylight of other high lighting conditions, but may correspond to an aperture diameter which is not sufficiently large enough to allow a sufficient amount of light through to the camera during night time or other low lighting conditions. Implementation of a larger aperture lowers the system f-ratio and helps alleviate low lighting problems, but this typically reduces image quality and decreases the depth of field or focus range of the optical system.
It has been proposed to implement an adjustable, rotatable aperture with variable diameter aperture settings in vision systems. Such rotatable apertures may provide enhanced image quality during periods of adequate lighting by using small aperture diameters, while providing improved lighting to the camera during lower light conditions by increasing the aperture size. Although implementation of a larger aperture during the low lighting conditions may reduce image quality, such an effect would not likely significantly degrade the image quality in a practical sense, since to the naked eye, there appears to be fewer spatial frequencies present in night time or other low lighting conditions. However, such a movable aperture comprises multiple movable components and mechanisms, which may significantly add to the cost and to reliability concerns of the device. Additionally, such a device may be of a greater size than is desirable for automotive applications, and may thus take up too much space within the tight space constraints of the vehicular application to be commercially practicable.
The present invention is intended to provide an electro-optic aperture for imaging sensors or cameras which is operable to vary the amount of energy being received by the sensors. An aperture according to aspects of the present invention can selectively allow a greater amount of light to pass therethrough when such energy or light would be useful to further illuminate a scene, such as when the intensity of the ambient decreases. As the lighting conditions increase, the aperture is operable to significantly attenuate a portion or region of the energy passing through the aperture, such that a smaller amount of light is received by the sensor. The smaller aperture or opening provides for improved focusing and greater depth of field of an image passing through the electro-optic aperture to the sensor during higher lighting conditions.
According to an aspect of the present invention, an imaging system for capturing an image of a scene comprises an imaging sensor, an electro-optic aperture and a control. The electro-optic aperture is operable to selectively attenuate light from the scene that is passing through at least one region of the aperture. The electro-optic aperture is positioned along an optic path from the scene to the imaging sensor. The control is operable to selectively activate the electro-optic aperture in response to an electronic signal. Preferably, the electro-optic aperture comprises an electrochromic medium, such as an electrochromic thin film, an electrochromic solid polymer film or an electrochromic solution, that attenuates the region of the light passing through the aperture in response to a voltage being applied to a pair of transparent electronic conductors of the electro-optic aperture. Optionally, the electrochromic medium is partially disposed on the transparent conductors and defines an opening in the electrochromic aperture that is void of the electrochromic medium. Alternately, one or both of the transparent electronic conductors, between which the electrochromic medium is disposed, can be zoned to create electrically isolated zones, such that each zone may be selectively energized to attenuate each selected zone of the aperture. Preferably, the imaging system further comprises a focusing lens with the electro-optic aperture being positioned between the focusing lens and the scene.
According to another aspect of the present invention, an imaging system for sensing an image of a scene comprises an imaging sensor, a focusing lens, an electrochromic aperture, and a control which is operable to energize the electrochromic aperture by applying a voltage thereto. The focusing lens is operable to focus the image of the scene onto the imaging sensor. The focusing lens is positioned along an optic path between the scene and the imaging sensor. The electrochromic aperture comprises at least one outer region and an inner region and is positioned along an optic path between the scene and the focusing lens. The electrochromic aperture is operable to attenuate energy received from the scene in the outer region when the aperture is energized, while allowing substantially non-attenuated energy to pass through the inner region.
In one form, the electrochromic medium may be disposed between transparent electronic conductors, which may comprise electrically isolated zones, each of which is selectively addressable by the control, such that at least one outer ring of the conductors is selectively energized, while an inner ring is de-energized. The control is then operable to selectively address the rings or zones in order to selectively energize the corresponding rings of electrochromic medium. In another form, the electrochromic aperture may comprise multiple layers of electrochromic medium and electronic conductors, each layer defining a different size opening through the aperture. The control may then selectively energize at least one of the layers to change the size of the opening in electrochromic aperture. Preferably, the control is operable to selectively energize the electrochromic aperture in response to the level of ambient light present in the scene.
These and other objects, advantages, purposes and features of this invention will become apparent upon review of the following specification in conjunction with the drawings.